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What Causes That Joint Popping Sound?

“What causes that joint popping sound?” 

Scientists solve the Joint Cracking debate

A University of Alberta research team that included Dr. Jerome Fryer, a chiropractor in British Columbia, uses MRI video for the first time to see what happens inside joints when they crack.

In a new study titled Real-Time Visualization of Joint Cavitation published April 15, 2015 in PLOS ONE (Public Library Of Science), researchers determine what happens inside finger joints to cause the distinctive popping sounds heard when cracking knuckles.

Cracking sounds emitted from human synovial joints have been attributed historically to the sudden collapse of a cavitation bubble formed as articular surfaces are separated. However, bubble collapse as the source of joint cracking is inconsistent with what happens within the joint during the joint cracking phenomenon. The first scientific study on the topic was published in 1947 and used serial radiography to visualize joint cracking when distraction forces were applied to metacarpophalangeal (MCP) joints. The films showed the presence of a clear space or cavity. This cavity, described by some as a bubble, has been thought to form as distraction forces decrease pressure within the synovial fluid to the point where dissolved gas comes out of solution. And the cracking sound was linked to the formation of the clear space. For the first time, they observed that the cause is a cavity forming rapidly inside the joint rather than bubble collapse. This theory stood for over two decades until 1971 when a group of researchers concluded that the popping sound coming from the joint when it was distracted was not from the formation of the space or bubble but its subsequent collapse.

The present study was conducted using a single subject while testing all 10 MCP joint individually. A cable was attached to the finger of interest and a distraction force was applied manually until the joint cracking occurred. As you can see in the image a cavity/ white image appears in the joint during distraction. Click here for MRI video

Still frames from the MRI joint cracking trial. A) Right 4th MCP in resting phase.B) Same MCP joint during joint separation. C) Immediately after joint cracking. D) The joint in the refractory phase immediately after removal of distraction forces.

The results of the April 2015 study offers direct evidence that joint cracking is the result of cavity formation within synovial fluid rather than the collapse of a pre-existing bubble because the cavity remains past the point of sound production.

One of the limitations of this study was that the forces used to create the joint cavitation were not measured and more research needs to be done. The lead author Greg Kawchuk, a professor in the Faculty of Rehabilitation Medicine states that the ability to crack you knuckles could be related to joint health. “By defining the process underlying joint cracking its therapeutic benefits, or possible harms, may be better understood.” Of interest is a 2011 study in the Journal of the American Board of Family Medicine titled Knuckle Cracking and Hand Arthritis. Researches in this study conclude that a history of habitual knuckle cracking does not seem to be a risk factor for hand osteoarthritis (OA). So there is your hard evidence that what your grandmother told you about cracking your knuckles was indeed and old wives tale.

The joint cavitation study is interesting to report on especially for the chiropractic profession whose practitioners get asked on a regular basis, “What causes that popping sound?” Chiropractors and their patients have been experiencing the joint popping that occurs during an adjustment for 120 years but it is with research like this that we can better explain what actually occurs during the process.

Douglas Krift, DC


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